Abstract
Prompt photon production in $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV Pb+Pb collisions has been measured by the ATLAS experiment at the Large Hadron Collider using data collected in 2011 with an integrated luminosity of 0.14 nb$^{-1}$. Inclusive photon yields, scaled by the mean nuclear thickness function, are presented as a function of collision centrality and transverse momentum in two pseudorapidity intervals, $|\eta| < 1.37$ and $1.52 < |\eta| < 2.37$. The scaled yields in the two pseudorapidity intervals, as well as the ratios of the forward yields to those at midrapidity, are compared to the expectations from next-to-leading order perturbative QCD calculations from JETPHOX. The measured cross sections agree well with the predictions for proton-proton collisions within statistical and systematic uncertainties. Both the yields and ratios are also compared to two other pQCD calculations, one which uses the isospin content appropriate to colliding lead nuclei, and another which includes the EPS09 nuclear modifications to the nucleon parton distribution functions.
Highlights
Prompt photons are an important probe for the study of the hot, dense matter formed in the high-energy collision of heavy ions
The two other JETPHOX calculations described in the previous section are shown, divided by the pp results: Pb + Pb collisions with no nuclear modification and Pb + Pb collisions with EPS09 nuclear modifications
It should be noted that where the data are more precise, in the central η region, the parton distribution functions (PDFs) modifications implemented in EPS09 compensate for the suppression at higher pT seen in the Pb + Pb calculations, giving cross sections similar to the pp case
Summary
Prompt photons are an important probe for the study of the hot, dense matter formed in the high-energy collision of heavy ions Being colorless, they are transparent to the subsequent evolution of the matter and probe the very initial stages of the collision. Where dNX/dpT is the yield of objects X produced in a pT interval, Nevt is the number of sampled minimum-bias events, TAA is the mean nuclear thickness function (defined as the mean number of binary collisions divided by the total inelastic nucleon-nucleon (N N ) cross section), and dσXpp/dpT is the cross section of process X in pp collisions for the same pT interval With this formula, one can make straightforward comparisons of yields in heavy-ion collisions, normalized by the flux of initial-state partons, to those measured in pp data, or calculated in pQCD. The ratios of the yields in the forward η region to those in the central η region (RFCη) are presented
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